The present invention relates in general to signal generators and more particularly to an apparatus for generating a periodic signal of adjustable frequency commencing an adjustable time following a triggering signal.
Sampling oscilloscopes were developed more than twenty years ago to observe small, fastchanging signals to which conventional oscilloscopes could not respond due to limited bandwidth or risetime characteristics. Sampling is a now well-known technique wherein a signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of an electrical signal during that period. Each sample taken in this manner is processed by electronic circuits and displayed as a dot on a cathode-ray tube (CRT) screen at an appropriate position corresponding to the relative timing and magnitude of the sample. Since the samples appear on the CRT display as dots, a large number of samples are required to accurately reconstruct a waveform. Generally speaking, sampling is most practical when the electrical signal is repetitive in nature since it is impossible to acquire all of the needed samples during a single cycle of all but relatively low frequency signals. Indeed, one of the advantages of sampling is that at least one sample can be acquired from each of a large number of cycles of a high frequency signal, and a representative waveform may be reconstructed and displayed therefrom.
Sampling modes are typified in accordance with the timing method used. Sequential sampling is a mode in which a waveform display is comprised of an orderly series of equally spaced dots. Random sampling is a mode in which successive dots may occur at what appear to be random horizontal positions because the sample timing and signal triggering are unrelated, although it must be pointed out that with random sampling the reconstructed waveform is defined because the dots are inserted into the display at substantially correct time positions.
Random sampling systems of the prior art have been adapted for sampling high frequency, repetitive waveforms by sampling a waveform at random points along several repetitive sections of the waveform. The resulting waveform display is then formed by ordering and graphically displaying sample data according to the relative sample time of each sample with respect to a triggering event, such as a zero crossing, occurring at the same point within each waveform section. The data acquired by this "equivalent time" method of sampling characterizes the waveform with a resolution equivalent to that which would be obtained if only a single section of the waveform were sampled at a much higher sampling rate. However since the sample timing is random, more samples must be taken to obtain a minimum resolution than would be required for periodic sampling.
In order for sequential sampling to be used for equivalent time sampling, the sampling times for each successive waveform section would have to be progressively skewed by a controlled phase angle with respect to a repetitive triggering event in a waveform to be sampled. Sequential sampling systems of the prior art sample waveforms periodically but are not adapted for equivalent time sampling because while the frequency of sampling timing can be precisely controlled, the point at which sampling begins during a waveform cannot be. In a sequential sampling system, sampling timing is typically controlled by a strobe generator which produces strobe signals for initiating sampling in response to a periodic input signal. What is needed is an apparatus for producing a periodic signal of adjustable frequency wherein the timing of the first cycle of the periodic signal with respect to a triggering signal can be accurately controlled with respect to a triggering signal.